FIX/ENH: attempt soft conversion of object series before raising a TypeError when plotting
DOC: add release notes/whatsnew
CLN: move PerformanceWarning to io.common and clean up
ENH/CLN: use @jtratner's decorator to check for raised warnings
Merge pull request #3912 from cpcloud/try-convert-when-plotting

FIX/ENH: attempt soft conversion of object series before raising a TypeError when plotting